In v0.9.0 RC10 the reference point of the bounding box (collision mesh) was changed from the body mesh origin to the vehicle centre of gravity. Some vehicles have an offset between these two points. In that case, the changed reference point can cause the vehicle to be "jacked up" and appearing floating on it's bounding box and stuck in place. You can check if that's the case by typing "show carbbox" into the ingame console for example. To fix it, you can adjust the y-component of the body.box_offset value in car.ini to be more positive, so the bounding box is moved higher up. The amount required will be identical to the cg.y value in car.ini.